Increasing the breakdown voltage of semiconductor junctions without reducing the doping levels is a common problem for new integrated circuit development.
Semiconductor junction breakdown is caused by the high electric field located at the junction. Various methods have been proposed to improve the breakdown for a given set of doping levels. These include grading the junction using multiple masks and/or multiple implants and using floating guard rings.
On heavily diffused junctions, multiple implants can cause problems due to unwanted vertical diffusion. To function properly, floating guard rings must be electrically isolated from the main junction; this is not possible with certain types of implants, such as n+ buried layers with overlying n-type epitaxial layer.